This invention relates to a laser system and, more particularly, to a scanning type laser system which emits a laser beam of uniform intensity distribution for medical treatment purposes.
The use of laser technology in the medical treatment of a variegated lesion such as a birthmark, freckle, or the like on the skin surface of a living body is known. Other methods are also known such as baking using electric drying and hardening, cell deconstruction using dry ice, surgery including cutting and slicing, and skin transplantation. However, all of these methods have disadvantages including a large invasion area, pain, long treatment periods, ineffectiveness and lengthy hospitalizations.
The method of baking the variegated lesion with a laser beam provides the advantage that, since a smaller invasion area is required, there is less pain. However, since the light intensity distribution perpendicular to the advancing direction of a laser beam is generally not uniform, it is difficult to obtain a maximum effect.
Contemporary laser treatments are generally classified into two methods. A first method emits a laser beam directly onto the variegated lesion, and a second method emits the laser beam onto the variegated lesion, after the beam has been passed through a light tramsmitting means such as an optical fiber. In the first method, the laser beam has a convex distribution of light intensity that is strongest at the center and gradually weakens towards the outside of the beam. In the second method, the laser beam emitted from the output end of a fiber has a far field pattern determined by the characteristics of the fiber, and the configuration of the light intensity distribution of the laser beam is complex, therefore it is in possible to obtain a beam of uniform distribution. Both of these methods result in irregular exposure of the laser beam.
To solve such problems, a kaleidoscope of the type, for example, described in an article of Grojean et al. (Review of Scientific Instruments, Vol. 51, No. 3, March, 1980, American Institute of Physics) can be employed. In the kaleidoscope, the light beam propagates while fully reflecting at the peripheral surface of the kaleidoscope, and the light intensity distribution is uniform at the beam emitting end of the kaleidoscope. By using a kaleidoscope which comes into intimate contact with a variegated lesion, the laser beam can be emitted to the variegated lesion with a substantially uniform distribution of light intensity. The configuration of the field wherein the laser beam is irradiated can be appropriately selected by fundling the kaleidoscopes. The laser beam emitted from the kaleidoscope beam emitting end is abruptly spread in the ambience to permit the abrupt decrease of a beam intensity, and the beam loses the coherency. Accordingly, this way of using the kaleidoscope has an advantage in that security is higher, even if the output beam should be mistakenly directed to another portion of the body, such as the eyes.
The kaleidoscope is a light transmitting medium in the form of a transparent square bar made of acrylic meterial or optical glass, or a hollowed square pipe made of metal. In the former kaleidoscope, the light beam is fully reflected at the peripheral surface of the bar by the difference of the reflectances of the bar and ambience. In the latter kaleidoscope, the internal surface is a mirror surface for fully reflecting the light beam, to randomly spread the beam. The both end faces of the kaleidoscope are flat.
Where the kaleidoscope is coupled to the ruby laser for medical treatment purposes, the laser output per unit area is 0.4 Joule/mm.sup.2, with specifications wherein the cross-sectional area of the beam emitting end of the kaleidoscope is 10 mm.times.10 mm, the output of the ruby laser (of 0.691 .mu.m in wavelength) is 40 Joule, and the pulse width of the pulse oscillating laser is 1 ms. This laser output is enough to destroy the variegated lesion when the lesion is irradiated with the laser beam while bringing the output end of the kaleidoscope into contact with the lesion.
In some cases, use of the argon laser is preferable to use of the ruby laser, since the distinguish length (i.e., the depth wherein 90% of the laser beam is absorbed) of the laser beam for a living organism and the light absorption characteristics of the variegated lensions differ with the types of laser beams. In general, the ruby laser is suitable for treatment of a brown or black variegated lesion and the argon laser is for a red variegated lesion. In this regard, the argon laser is of the continuous output type, with a wave length of about 0.5 .mu.m. In treating the variegated lesion, a laser beam of approximately 4 W is led to the lesion by an optical fiber which is about 1 mm in core diameter. The output end of the argon laser is 1 mm .phi. and is small, being limited in the following ways. First, since the area of the variegated lesion is generally at least 1 cm .phi., to irradiate the entire area of the variegated lesion with the argon laser, the output end of the optical fiber must be displaced on the lesion several times, for a complete irradiation. Secondly, through the irradiation of several steps, it is impossible to uniformly irradiate the entire surface of the lesion, since the configuration of the cross section of the optical fiber is circular.
To obtain uniform irradiation, the use of the kaleidoscope in argon laser irradiation may be considered effective. Actually, however, use of the kaleidoscope increases the cross-sectional area of the laser emitting end, resulting in a reduction of the output density. To be more specific, the argon laser provides a continuous oscillating output of about 4 W. For the optical fiber with an output end of 1 mm .phi., the output density of the argon laser is 509 W/cm.sup.2. If the kaleidoscope with a light emitting end area of 10 mm.times.10 mm is connected to the argon laser, the output density is 4 W/cm.sup.2, which value indicates a 0.79% of that in the case of optical fiber use. Such a low output density of 4 W/cm.sup.2 would preclude the treatment of the variegated lesion, since in the laser beam treatment, with an instantaneous absorption of an extremely high energy to the variegated lesion, only the variegated lesion is destroyed while normal cells are intact.
For the above reasons, a laser system capable of uniformly irradiating the variegated lesion with a laser beam, at a high output density and over a wide area, is very much in demand. Thus, it is impossible to widen the cross-sectional area of the beam emitting end by connecting the kaleidoscope to the laser system at a low output density, such as the argon laser.
Further, in variegated lesion treatment, it is necessary to irradiate only the lesion with the laser beam, not that portion of the body containing normal cells. Actually, whether or not the cells in the irradiation field are normal or not depends on the judgement of an operator, which inevitabaly entails some uncertainty.